1. Field of the Invention
The present invention relates to electromagnetic exciters that may be incorporated into cellular phones or other devices to generate sound or vibration in response to an electric signal.
2. Description of the Background Art
Conventionally, mobile communications devices such as cellular phones and personal digital assistants (PDAs) are arranged to inform the user of an incoming call by selectively generating beep sound or melody sound, or vibrating the housing of the device instead of producing sound. For this purpose, these communications devices have a combination of a small speaker and a vibrator that generates vibration by rotating an eccentric weight with a small motor. Incorporating both a speaker and a vibrator into such a communications device, however, is disadvantageous from the viewpoint of achieving size and cost reduction of the device. Under these circumstances, use has recently been made of an electromagnetic exciter that can generate both sound and vibration by itself [see, Japanese Patent Application Publication No. 2003-251278].
FIGS. 3 and 4 each shows such a conventional electromagnetic exciter. The illustrated electromagnetic exciter has a synthetic resin casing 12 with a cylindrical side wall 12a. A diaphragm 13 for generating an acoustic output is secured to the upper peripheral edge of the casing 12. A voice coil 14 is attached to the diaphragm 13 and inserted into a magnetic gap g formed in a magnetic circuit assembly 18. The magnetic circuit assembly 18 includes a cylindrical cup-shaped outer yoke 16 formed from a high-permeability material and an axially magnetized cylindrical permanent magnet 15 provided in the outer yoke 16 and secured to the bottom wall of the outer yoke 16. The magnetic circuit assembly 18 further includes an inner yoke 17 formed from a high-permeability material and secured to the upper end of the permanent magnet 15. A top cover 20 is attached to the top of the casing 12. A back cover 21 is attached to the bottom of the casing 12.
The outer yoke 16 is connected to the casing 12 through a suspension 19. Thus, the outer yoke 16 is supported by the suspension 19 so as to be vibratable in its axial direction.
When the voice coil 14 is excited by a driving signal, the voice coil 14 and the magnetic circuit assembly 18 vibrate relative to each other by interaction between the voice coil 14 and the magnet 15. When the driving signal is of a relatively high frequency in an audio-frequency region, the voice coil 14 mainly vibrates, causing the housing of the electronic device to vibrate through the diaphragm 13. When the frequency of the driving signal is low, the magnetic circuit assembly 18 mainly vibrates, and this vibration causes the housing of the electronic device to vibrate through the suspension 19 and the casing 12.
The outer periphery of the outer yoke 16 is integrally formed with a ring-shaped additional weight 16a made of a high-density magnetic material, e.g. iron, or a tungsten alloy, to increase the weight of the magnetic circuit assembly 18, thereby lowering the natural frequency of the magnetic circuit assembly 18 and reducing the amplitude of the vibration of the magnetic circuit assembly 18, and thus enabling the whole electromagnetic exciter to be reduced in size (thickness) correspondingly.
In the electromagnetic exciter shown in FIG. 4, an additional weight 16b prepared as a discrete member from the outer yoke 16 is connected to the outer yoke 16.
The above-described conventional electromagnetic exciter suffers from the following problem.
The suspension 19 is secured with its radially outer end portion inserted into the cylindrical side wall 12a of the casing 12 by insert molding. Accordingly, the cylindrical side wall 12a needs to have an increased thickness in order to support the suspension 19. If an attempt is made to increase the thickness of the cylindrical side wall 12a while keeping the external shape thereof constant, the internal volumetric capacity for accommodating the magnetic circuit assembly 18 is reduced, and the extent to which the volume of the additional weight may be increased is limited correspondingly.